There exist throughout the world deposits or reservoirs of heavy oils and bitumen which, until recently, have been ignored as sources of petroleum products since the contents thereof were not recoverable using previously known production techniques. While those deposits that occur near the surface may be exploited by surface mining, a significant amount of heavy oil and bitumen reserves may occur in formations that are too deep for surface mining, typically referred to as “in situ” reservoirs or deposits because extraction must occur in situ or from within the reservoir or deposit. The recovery of heavy oil and/or bitumen in these in situ deposits may be hampered by the physical characteristics of the heavy oil and bitumen contained therein, particularly the viscosity of the heavy oil and/or bitumen. While there is no clear definition, heavy oil typically has a viscosity of greater than 100 mPa/s (100 cP), a gravity of 10° API to 17° API and tends to be mobile (e.g. capable of flow under gravity) under reservoir conditions, while bitumen typically has a viscosity of greater than 10,000 mPa/s (10,000 cP), a gravity of 7° API to 10° API and tends to be immobile (e.g. incapable of flow under gravity) under reservoir conditions. The above noted physical characteristics of the heavy oil and bitumen (collectively referred to as “heavy oil”) typically renders these components difficult to recover from in situ deposits and, as such, in situ processes and/or technologies specific to these types of deposits are needed to efficiently exploit these resources.
Several techniques have been developed to recover heavy oil from in situ deposits, such as stream assisted gravity drainage (SAGD), as well as variations thereof using hydrocarbon solvents (e.g. VAPEX), steam flooding, cyclic steam stimulation (CSS) and in-situ combustion. These techniques involve attempts to reduce the viscosity of the heavy oil so that the heavy oil and bitumen can be mobilized toward production wells. One such method, SAGD, provides for steam injection and oil production to be carried out through separate wells. The SAGD configuration provides for an injector well which is substantially parallel to, and situated above a producer well, which lies horizontally near the bottom of the deposit. Thermal communication between the two wells is established, and as oil is mobilized and produced from the producer or production well, a steam chamber develops. Oil at the surface of the enlarging steam chamber is constantly mobilized by contact with steam and drains under the influence of gravity.
An alternative to SAGD, known as VAPEX, provides for the use of hydrocarbon solvents rather than steam. A hydrocarbon solvent or mixture of solvents such as propane, butane, ethane and the like can be injected into the reservoir or deposit through an injector well. Solvent fluid at the solvent fluid/oil interface dissolves in the heavy oil thereby decreasing its viscosity, causing the reduced or decreased viscosity heavy oil to flow under gravity to the production well. The hydrocarbon vapour forms a solvent fluid chamber, analogous to the steam chamber of SAGD.
It has been recognized, however, that these prior means used for the recovery of heavy oil from subterranean deposits need to be optimized.